All the knowledge I had about viruses is that they cannot live without host.But what exactly an inactive virus ( either by drying for sufficiently long time or by chemicals) means ? Does it mean that it is permanently destroyed for future and cannot be made active even by supplying again the lost nutrients or it means a sort of dormancy? When people say "inactive" , can it be taken as permanent destruction without being any danger in near future? I am asking for any virus or hiv in particular.Replies are appreciated.

Viruses do not really "live" at all - when they are outside the host cell, they are not doing anything except drifting around and waiting to meet a suitable target cell to infect. So, viruses do not need a host to live, but they need a host to make copties of themselves.

The term "inactive" or "inactivated" virus typically means the virus has become or been made permanently inactive. This can happen in various ways, but normally it means that some parts of the virus get destroyed by chemicals, physical factors or biological mechanisms. It requires permanent changes to the proteins, lipid envelope or the nucleic acids of the virus, thus rendering the viral particle inactive in terms of the ability to infect its target cells or to replicate inside them.

Typical inactivating agents (such as detergents) denature viral proteins, dissolve the lipid envelope or break down nucleic acids. Heat and light (especially UV light) can cause some of these events to happen as well. Many organisms also have protein and nucleic acid degrading enzymes within their cells that are used to splice the viral particles into pieces before they infect the cells.

Genetically engineered viruses can be inactive in such a way that they cannot replicate in the host cell, but they can enter the cells and deliver genetic material into them - these can be used as gene delivery vectors.

HIV is pretty typical as a virus: it is quite fragile and gets easily destroyed (inactivated) by many common environmental factors, as well as by the common detergents and similar chemical substances. It stays alive in body fluids and gets quickly inactivated outside the body.

Some (mostly environmental) viruses are more durable and can stay alive in moist environments for longer periods of time; there are viruses in seas and fresh water, as well as in the soil, but even there they require the presence of their host organisms and would disappear quite soon if they cannot find any hosts.

Viruses without lipid membrane are more resistant to some chemicals whose primary way of action is dissolving lipids; DNA-containing viruses are generally more durable than RNA viruses.

Thanks "biohazard" for detailed explanation but I am still not clear. Are they get permanently destroyed or there is still possibility after their inactivation.Also if only drying can cause complete inactivation.

However, in theory you could have various viral particles which themselves would all be inactive (unable to cause disease), but could spontaneously form an infectious virion. But if you have, for example, a product that contains inactivated viruses (like some vaccines do), then they must be made inactive in such a way that they cannot form infectious particles anymore - they are permanently "dead".

When you say , viral particles are inactive (unable to cause disease), then I guess you are talking of inactivated virus outside host and thus unable to infect but my question is that if these inactive viral particles are again supplied with their requirements (say , they are injected into bloodstream of their host, which was their ideal environment) , can they be infectious to this new host ?

If the answer to this is No , then my second question is : if drying of the fluid containing viral particles is only sufficient to cause such irreversible inactivation ?

bioshare wrote:When you say , viral particles are inactive (unable to cause disease), then I guess you are talking of inactivated virus outside host and thus unable to infect but my question is that if these inactive viral particles are again supplied with their requirements (say , they are injected into bloodstream of their host, which was their ideal environment) , can they be infectious to this new host ?

If the answer to this is No , then my second question is : if drying of the fluid containing viral particles is only sufficient to cause such irreversible inactivation ?

The term "inactive" applies to viruses both inside and outside the host. Basically, what happens in such inactivation event is permanent cross-linking of viral proteins, dissolution of the lipid membrane and/or splicing of the nucleic acids. Artificially, some of these events can be countered - for example a new lipid envelope can be given to an intact viral protein capsule or an undamaged vrial genome can be repacked into a new protein capsule, but these require sophisticated laboratory environments. If you inject inactivated viruses into a host, the viruses cannot cause infection - if they can, they were not inactive.

For majority of human viruses simple drying is enough to permanently inactivate the viruses in such a way that they do not cause disease even if re-injected to the host later. Even viruses that spread outside the host (like flu viruses by aerosoles) die quite often if exposed to dry air. In moist surroundings and protected from light many of them last longer. For example, many enteroviruses can stay alive in the soil and water for longer periods of time.

Julie5 wrote:So, if inactive means, effectively, 'dead' what is the term that is used for when a virus is not in a host, just hanging out looking to mug one? Is it 'dormant'?

Also, what does the term 'attenuated' mean in terms of vaccines? Does it mean inactive (dead) or simply a less virulent strain in some way?

Many thanks!

'Dormant' is quite a good word to describe viruses outside the host (or more precicely, the host cell). However, that would mean that all viral particles are "dormant" except when they bind to their target receptor on the cell surface and inject their genome and possible enzymes inside the cell. In my opinion you could just as well call any non-inactive viruses 'active' viruses, because they are active so that they can recognize the target cell and infect it.

Maybe think viruses as biological bombs: a bomb can be disarmed (= inactivated) and a disarmed bomb will not explode upon finding its taget. An intact bomb is 'active', even though there is nothing actively happening inside the bomb - there is activation only briefly when it hits the target. The same goes with viruses: nothing happens, except when it hits the target.

Now, an attenuated virus is a virus that has been manipulated in a way that some key element(s) of its virulence have been removed. You could think an attenuated virus as a practice bomb or munition: it has an intact casing (=protein capsule) and fuse (=proteins that recognize the target cell) but it lacks the most or all of the explosive material (=parts of the genome required for infection, or some enzymes that help the virus to be copied inside the target cell). So, the practice bomb makes a loud bang but no harm is done, and the attenuated virus may sometimes infect the cells around its injection site, but is unable to spread the infection. Or more often, it simply cannot enter any cells, because its 'targeting system' has been altered as well.

In practice, attenuated viruses are usually mutated in another host organisms so much that they cause the original, protective immune response in human, but cannot spread in humans because they have mutated to use that other host organism (their genome and proteins made by the genome have been altered from the original ones).

I think those are all correct, but as far as I know the term 'dormant' is not commonly used. This is mostly because there is no really a need to distinquish active and dormant viruses - after all, even a dormant virus is 'active' in such a way that it is always ready to infect a cell if it find suitable one. And even an 'active' virus is not doing much: it simply gets triggered upon binding on its target receptor, automatically injecting its contents to the target cell and after that the host does the work (although some viral enzymes may take part in the processes, but they are not inside the virus any more).

In my opinion, you should be just fine with the terms 'active' and 'inactive' (or, 'live' and 'dead', but those are not quite as good as viruses are not really 'alive' as we usually define it). When the virus gets inside the host cell, it actually gets dismantled there: it unloads its genome and the capsule proteins are discarded. If the virus contains its own enzymes, those are also transported to the cytoplasm. Actually, you cannot really say the infecting virus is 'active' when its inside the host, because it is no more! The new viruses produced by its genome, in turn, are active, get ejected from the host cell (often killing it) and then start a new cycle of host-searching.

So, also here you could perhaps use the 'biological bomb' analoque: when the virus infects the cell, it 'detonates' - that is, breaks down into pieces, which then force the host cell to manufacture more viral pieces and assemble them as brand new viral particles, 'bombs' ready to infect new cells.

Perhaps the most precise terms would be 'intact' and 'broken' (or something similar) - an intact virus is a virus that is in one piece, ready to infect and briefly undergoes activation upon binding its target before it is dismantled. A broken virus, in turn, is a virus that has lost the function of one or more essential parts and cannot cause infection. Maybe add a third term 'attenuated', which means a virus that is intact, but in wrong environment to work properly.